As a conventional type of photo-receptor for electrophotograghy, inorganic photo-receptor having a photosensitive layer whose principal component is an inorganic photoconductive compound such as selenium, zinc oxide, cadmium sulfide, and silicone, has been in wide use. However, these photo-receptors are not necessarily satisfactory in terms of sensitivity, thermostability, moisture resistance, and durability. For example, when selenium is used as a photo-receptor, it easily deteriorates when it is crystallized, which can cause difficulty in manufacturing selenium. Also, it can be crystallized by heat and fingerprints. Cadmium sulfide has problems with moisture resistance durability, and zinc oxide has problems with durability.
To overcome the shortcomings inherent in the foregoing inorganic photo-receptors, research and development has actively been made to develop organic photo-receptor having organic photoconductive layers whose primary components are a variety of organic photoconductive compounds. For example, Japanese Patent Publication No. 10496/1975 discloses an organic photo-receptor having a photosensitive layer containing poly-N-vinylcarbazole and 2, 4, 7-trinitro-9-fluorenone. However, this photo-receptor is not necessarily satisfactory in terms of sensitivity and durability. To improve these shortcomings, attempts have been made to allot different substances to different functions, i.e., carrier generation and carrier transport, thereby to develop organic photo-receptors of higher-performance. This so-called function-separating type of photo-receptors has been the subject of many studies because the respective materials can be selected from wide variety of compounds and, for this reason, it has been expected to obtain photo-receptors with arbitrary proparties.
In the function-separating type photo-receptors, numerous number of compounds have been proposed as carrier-generation substances. As an example in which an inorganic compound is used as a carrier-generation substance amorphous selenium as disclosed in Japanese Patent Publication No. 16198/1968 may be mentioned. This compound is used in combination with an organic photoconductive compound, however, it cannot overcome the shortcomings of an amorphous selenium, which is liable to be crystallized by heat, leading to the deterioration of its properties as a photo-receptor.
Many other proposals have been made for photo-receptors for electrophotography using organic dyes and organic pigments as carrier-generation substances. For example, Japanese patents Open to Public Inspection Nos. 22834/1979, 73057/1980, 117151/1980, and 46237/1981, refer to the use of bis-azo compounds in the photosensitive layer. Those bis-azo compounds are, however, not necessarily satisfactory in terms of sensitivity, residual electric potential or stability in the repeated use, and in vie of its limited selection range of carrier transport substances. Thus they cannot fulfill the broad requirements of the electrophotographic process.